gO SCIENCE PROGRESS 
7.e. consist of a mixture of equal parts of the dextro- and levo- 
rotatory varieties. The real synthesis of the natural compounds 
is, therefore, only attained when the synthetical substances 
have been separated into their two constituents. Two methods 
may be used for this purpose: the biological method— 
destroying the one isomer by means of micro-organisms, such 
as Penicillium glaucum and Aspergillus mger—had been used 
by Schulze and Bosshard to obtain leucine and glutamic acid, 
and by Engel to obtain aspartic acid; but the chemical method 
of combining with optically active compounds to form salts 
and fractionally crystallising these salts had not been employed 
with any success. This was probably due to the small affinity 
which the simple monoamino acids have for acids and bases. 
The aminodicarboxylic acids, on the other hand, are moderately 
strong acids, which combine with bases to form salts of this 
kind, but the desired results were not achieved in the attempts 
thus made to separate the optically active forms. 
Fischer found that by preparing the benzoyl] derivatives of 
the amino acids, their acidic character was greatly increased 
and that they then formed with the optically active bases— 
strychnine, brucine, and morphine—stable salts which were 
soluble with greater difficulty than the amino acids and more 
easily isolated. Hippuric acid, or benzoylglycine, has been 
known for a long time ; the derivatives of the other amino acids 
are similar in constitution. The ordinary process of benzoylating 
by the Schotten-Baumann method in the presence of excess of 
caustic soda gave poor yields of the benzoyl derivatives of the 
amino acids, but by substituting sodium bicarbonate for the 
caustic soda and using a large excess of benzoylchloride 
Fischer obtained excellent results, and succeeded in preparing 
the optical isomers of alanine, leucine, tyrosine, phenylalanine, 
aspartic and glutamic acids, as also of a-aminobutyric acid and 
a-amino-#-caproic acid. 
For the preparation of large quantities of optically active 
amino acids, such as is required in the synthesis of the proteins, 
the benzoyl method for the separation of the isomers was not 
found to be convenient on account of the fact that the recon- 
version into the amino acid was often carried out with difficulty, 
since it required a long time and large quantities of acid 
(100 times the quantity of 10 per cent. hydrochloric acid) to 
completely hydrolyse the compound. Fischer had therefore to 
